1. Field of the Invention.
This invention relates to semiconductor processing equipment and, more particularly, to systems and methods for determining if a plasma etching process is out of tolerance.
2. Prior Art.
Plasma etchers are used in semiconductor processing to avoid undercutting of a layer being etched and provide straight vertical edges. An important step in a plasma etching process is stopping of the etching process after the layer being etched has been removed but before the next layer is removed. Determination of the point at which to stop further etching is called "endpoint detection." Etching beyond the endpoint may cause unwanted removal of desired portions of the next layer, such as, for example, thin gate oxides. As the device geometries of semiconductor devices continue to decrease in size, the need for more accurate endpoint detection increases.
Etching a particular layer on a semiconductor wafer with a RF plasma generates light energy at a particular wavelength where that wavelength corresponds to the material being etched. An optical sensor is used to monitor the intensity of the light energy at that particular wavelength corresponding to the material being etched. An endpoint is determined to have been reached when the optical sensor indicates that the light energy at the particular wavelength has fallen below a specific level. An endpoint signal is then generated to stop etching of the particular layer on the semiconductor wafer.
FIG. 1 diagrammatically shows a semiconductor etching system 10 which uses a conventional endpoint detection technique. An etching chamber 12 contains a RF plasma 14 which is generated therein for etching away selected unmasked portions of a film layer 16 previously formed on a wafer 18 contained within the chamber 12. The plasma etching process typically removes selected unmasked portions of the film layer 16 to expose an underlying layer 20, such as, for example, a portion of the top surface of the wafer 18 or another layer formed on the wafer.
As previously mentioned, etching away selected portions of a particular layer on a semiconductor wafer with a RF plasma generates light energy at a particular wavelength which corresponds to the specific material being etched. Different etched materials generate different wavelengths. In order to view the particular wavelength, the etching chamber 12 also includes a window 22 through which an optical detector of a detector and wavelength sampler unit 24 views the RF plasma 14 in the chamber 12. The optical sensor, or detector, is used to monitor the intensity of light energy at a particular wavelength corresponding to the particular material being etched away from the unmasked top surface of the wafer 18 in the etching chamber 12. The optical sensor provides a continuous intensity signal corresponding to the intensity of the light energy at the particular wavelength for the material being etched from the unmasked top surface of the wafer 18. A wavelength intensity sampler periodically samples the continuous intensity signal and provides a series of stepped sample signals to a wavelength-intensity-to-digital signal converter 26, which provides a periodic series multi-bit digital output code word, or counts, corresponding to each wavelength sample. A count is taken at intervals ranging, for example, from one-half to one second.
The counts, or multi-bit digital output code words, of the wavelength-to-digital converter 26 are then processed by software in a computer system 28. The computer system 28 processes the multi-bit digital output code words to determine an endpoint signal.
FIG. 2 illustrates that the envelope of the counts, or digital output signals, from the converter 26 which are processed by the computer 28 can be visualized in a graphic form as a curve 30 of counts vs. time. The typical endpoint curve 30 of FIG. 2 is for an etched material which has a characteristic wavelength which decreases in intensity as the material is etched away. This curve 30 can be displayed on a display screen. An endpoint level 34 is set as a predetermined percentage drop from a peak, or maximum, count 36. A typical predetermined percentage is 90 per cent. When the selected portions of the film layer 16 are removed, the intensity at the selected wavelength decreases. When the light energy at the particular wavelength corresponding to the material being etched has fallen below the endpoint level 34, an endpoint signal is generated and sent on a signal line 38 from an output terminal of the computer system 28 to an input terminal of the etching chamber 12. The endpoint signal stops etching of the particular layer on the semiconductor wafer.
FIG. 2 illustrates that the time at which the endpoint level 34 is reached occurs at a time t.sub.1. If the endpoint signal is weak, the endpoint time is extended and results in over-etching of a wafer.
Current endpoint systems only evaluate percentage changes from a preset mean or average reference point so that problems with an etching process are not immediately identified.